The present invention relates generally to automation of the processing of logs, cants and other wooden workpieces and more particularly to methods and apparatus for scanning and dimensionally analyzing such workpiece to maximize the yield of wood products therefrom. Although the invention is described in connection with processing of cants, it is also applicable to sawing and turning logs.
Many modern lumber mills now employ some type of automatic scanning equipment which feeds in formation regarding workpiece dimensions to a data processing device for analyzing the dimensional data to control cutting mechanisms. Broadly, the prior systems suffer from two disadvantages. First, such systems conventionally seek to maximize the volume of salable wood products from each log, but do not necessarily produce the most economical mix of wood products. Second, the prior systems obtain huge amounts of data about the workpieces, necessitating very large memory and computing capacity to process. Consequently, such systems are very expensive, often too expensive for small mills. It would be preferable to have a method and apparatus for controlling the processing of wooden workpieces which is capable of maximizing the economic yield in salable wood products, and yet is simpler and less expensive than those systems provided by the prior art.
In edging a cant to produce dimension lumber, any irregular margins of the cant, or wane, are cut off and the cant is sawn lengthwise into boards of various widths. A cant is typically trapezoidal in cross section and has irregular side faces, but may be square along one or both side faces, depending on how the cant is sawn from a log. In the former case, the cant is edged to square it up, by removing its irregular, triangular cross-section margins or wane. Then, in both cases, the cant is sawn lengthwise into boards of varying widths. Conventionally, the configuration of the cant is analyzed to determine the widthwise positions of several sawlines which will provide the combination of board widths that maximizes the volume of lumber produced from the cant. Similar analytic techniques are employed in sawing a log into cants.
However, the conventional optimization methods do not take into account the economic value of the lumber produced as a function of its dimensions. Also, they do not taken into account the variations in value of a board of a given dimension as a function of the grade of the lumber. The grade of a given piece of lumber is affected by a number of variables, including the presence or absence of knots, splits and rot in the wood. The grade also varies with the amount of wane, or corner truncation, remaining on the finished lumber. Conventional optimization techniques largely ignore these economic factors.
The customary approach to edging a cant is to remove essentially all of the wane, ignoring variations in the inherent grade of the wood due to knots, splits and rot. If the cant is trapezoidal, the edging process results in the loss of a triangular or trapezoidal cross section strip of wood from each side of the cant sufficient to square up the entire thickness of the cant over its entire length. This approach, in effect, saws each cant in a manner commensurate with producing the highest grade of lumber, without taking into account inherent characteristics of the wood that might prevent the resultant lumber from achieving such high grade.
Thus, by ignoring economic value as a function of both width and grade, significant amounts of otherwise marketable wood are lost. If, due to variations in market conditions, lumber of a certain dimension has a higher value on a volumetric basis than lumber of other dimensions, it is preferable to cut as much lumber as possible to the higher value dimension to maximize economic yield. However, the aggregate economic value of the wood products produced from a cant or log is not maximized by maximizing only the gross volume of lumber produced from a cant of a given width, without regard for the relative values of different dimensions of lumber. And if, because of inherent characteristics of the wood, the lumber produced could not possibly exceed a given grade level, cutting off more wane than is commensurate with that grade level wastes useful wood.
One of the most commonly-used system for scanning a cant preparatory to edging, employs a method of shadow scanning the cant as disclosed, for example, in U.S. Pat. No. 3,970,128 to Kohlberg. This system provides for a pair of illumination sources positioned above and to each side of a conveyor along which a cant is conveyed to illuminate the sides of the cant at a downward angle. A scanner is positioned directly above the cant to receive light reflected upward from the sides of the cant. The light sources are operated alternately so that one side is shaded while the other side is illuminated. In this way, the longitudinal edges of the top of the cant surface are defined by distinct shadow lines which are readily detected by the scanner. Scanning is synchronized with the alternate lighting of the illumination sources. The scanning outputs are fed to a computer for calculation of an optimum distance between two straight edging cuts and an optimum orientation of those cuts to convert the cant to a standardized finished piece of lumber with minimum wastage of material. U.S. Pat. No. 3,806,253 to Denton applies this technique to the scanning of logs.
A second technique, applied in U.S. Pat. No. 4,097,159 to Strandberg to scanning cants and in U.S. Pat. No. 4,192,613 to Hammar to scanning logs, aligns the light source and scanner on opposite sides of the workpiece so that the workpiece interrupts the transmission of light from the source to the scanner. The position of the interruption is detected by the scanner and provided to a data processor which analyzes the dimensions of the silhouette of the workpiece to control its processing.
A third scanning technique is disclosed in U.S. Pat. Nos. 4,158,507 to Himmel; 4,264,208 to Haberl, et al.; 4,294,149 to Olsson and 4,300,836 to Holmes, et al. Generally speaking, this technique employs scanning a beam across back and forth or lengthwise along the workpiece, using a beam pivoting device, and measuring the dimensions of the surfaces of objects scanned by triangulation or reflected light intensity techniques.
A fourth technique disclosed in U.S. Pat. No. 4,188,544 to Chasson and various patents cited therein uses a television camera and a fan-line laser projected downwardly onto a workpiece at different acute angles for detecting the intersection line of a plane of light produced by the laser and the workpiece and calculating dimensions of the workpiece surface by using known distances and geometric relationships. Systems similar to that of Chasson are also disclosed in U.S. Pat. Nos. 4,086,496 to Berry; 4,186,310 to Maxey and 4,196,648 to Jones, et al. These systems all use a separate set of scanners and light source at each scanning interval along the cant.
As mentioned above, all of the foregoing lumber processing control systems analyze the dimensions of the workpiece and control their processing to maximize the volume of usable wood products to be produced from the workpiece. None of them are known to take into consideration the relative economic value or grade of the products. For the most part, the analytic methods employed in these systems involve mapping the entire surface of the workpiece. Therefore, sufficient memory (e.g., 256,000 bytes) must be provided in the data processor to store three-dimensional spatial coordinates of the workpiece. Substantial computing capacity, such as is provided by a main frame or minicomputer, is required.
Also, data acquisition time in such systems is long, one second or more. This does not leave time for much analysis when trying to edge 15 to 20 cants per minute. Speed is especially important when handling relatively narrow cants, under 16 inches, because of the need for high volume throughput to economically edge narrow cants. Long data acquisition processing time virtually precludes application of prior automated scanning and analysis techniques to small-log lumber mills. Typically, edger operators in such mills, unaided by any automated analysis, only have time to briefly view each cant, make a snap decision and push a button. The resultant decisions are typically not optimal.
Referring to the aforementioned Chasson patent, the amount of memory and computing capacity required can be reduced somewhat by sampling the dimensions of the workpiece at intervals spaced along the length of the workpiece. However, this approach sacrifices substantial accuracy. It can miss significant variations in the contours of the workpiece between sampling intervals, which are conventionally positioned six inches or more apart. And the time and cost savings are too little to enable use of low cost microcomputers for conducting extensive analysis of edging options.
Accordingly, there remains a need for a scanning and analysis system which is capable of accurately characterizing the dimensions of a workpiece, without requiring storage and processing of vast amounts of data, and controlling the processing of the workpiece so as to maximize the economic value of the wood products produced therefrom.